"I wonder why nVidia offers these type of configurations of it's graphics cards, in this case > GT730." - Market demand. As for the reason for the 3 configurations you are asking about, market demand for a product at a specific price point, and the configurations were choose to accomplish that
– RamhoundOct 11 '17 at 13:57

1 Answer
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I wonder why 64 bit version has 2GB and not 1GB? >While 128 bit version has 1GB.

This is almost certainly because the GPU has two memory controllers.

You get either two 64-bit channels capable of using 1GB of memory each, or one 128-bit channel capable of using a single 1GB memory area.

I wonder why GDDR5 version has 1GB and not 2GB? >While DDR3 version has 2GB.

Probably because the extra bandwidth supplied by GDDR5 requires some internal multiplexing in the GPU which limits it to one memory channel. The real reason would only be known to GPU designers and Nvidia themselves.

Also I wonder if 64bit 4GB performs same as 128bit 2GB card? or 64bit 2GB perfors same as 128bit 1GB Card?

No. The "bitness" of the memory affects the effective memory bandwidth available. Look at the figure just below the memory interface width.

Graphics tasks tend to be incredibly memory bound, more memory bandwidth generally means it will perform better. Even then there may be tasks where a lower bandwidth card with more memory might perform better than a card with less memory but higher bandwidth.

You are down comparing the weeds amongst several (quite frankly) very poor cards and I seriously doubt you are going to see a massive performance difference between them.

So when specification say there is 64 bit bus, that means that there are 2 of them? Two separate 64 bit buses? On every card?
– George G.Oct 11 '17 at 11:47

Not on all of them, no. But there is an implication in offering a part that is capable of both. Quite honestly if you are looking to buy one of these cards then just go out and buy what you can. The performance isn't going to be particularly spectacular on any of them as they are only just a step above being in the bargain bucket range.
– Mokubai♦Oct 11 '17 at 11:53